harden abbr against undef vars

This is only needed if we make undefined vars an error as proposed in
issue #4163 but it's a good idea anyway.

share/functions/abbr.fish

@@ -96,8 +96,10 @@ function abbr --description "Manage abbreviations"
             end
 
             # Bail out early if the exact abbr is already in
-            contains -- "$mode_arg" $fish_user_abbreviations
+            set -q fish_user_abbreviations
+            and contains -- "$mode_arg" $fish_user_abbreviations
             and return 0
+
             set -l key $mode_arg[1]
             set -e mode_arg[1]
             set -l value "$mode_arg"
@@ -186,6 +188,10 @@ function __fish_abbr_get_by_key
         echo "__fish_abbr_get_by_key: expected one argument, got none" >&2
         return 2
     end
+
+    set -q fish_user_abbreviations
+    or return 1
+
     # Going through all entries is still quicker than calling `seq`
     set -l keys
     for kv in $fish_user_abbreviations

tests/abbr.in

@@ -11,7 +11,7 @@ abbr __abbr1 alpha beta gamma
 abbr | grep __abbr1
 
 # Replacing
-abbr __abbr1 delta 
+abbr __abbr1 delta
 abbr | grep __abbr1
 
 # -s and --show tests